The purpose of this research is to examine comparatively the functions of reproductive hormones, including estrogen and progesterone, in the regulation of sexual and social behaviors. The roles of exogenous (male-related) stimuli and hormones in the mediation of the induction, maintenance and termination of female sexual behavior will be investigated. Concurrently we will analyze the behavioral endocrinology of pair bonding or monogamy and intraspecific aggression, which is altered by sexual experience and/or familiarity. Behavioral procedures that permit long-term observations, hormone assays and exogenous hormone treatments will be employed to investigate these problems. The initial phases of this work will investigate the monogamous mammal, Microtus ochrogaster (the prairie vole). Later phases will compare the behavioral physiology of this species to the sympatric, nonmonogamous Microtus pennsylvanicus (or other species). These microtine rodents are important models for population biology and field data are available regarding their patterns of social organization.